Sector 106 3BHK: Uncovering hidden costs and negotiation secrets

Sector 106 – researching this area for 6 months. Planning to upgrade my 2BHK to a 3BHK, maybe Aask For Realty. For investors, what's realistic negotiation on a 1.2Cr 3BHK? Is 5-7% wishful thinking? What hidden charges surface at possession, beyond stamp duty? Are builders adding sneaky PLC or maintenance fees?